.\" -*- nroff -*-
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" Copyright (c) 2014 Cisco Systems, Inc.  All rights reserved.
.\" $COPYRIGHT$
.TH MPI_Wtime 3 "Jun 10, 2020" "4.0.4" "Open MPI"
.SH NAME
\fBMPI_Wtime\fP \- Returns an elapsed time on the calling processor.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
double MPI_Wtime()

.fi
.SH RETURN VALUE
.ft R
Time in seconds since an arbitrary time in the past.

.SH DESCRIPTION
.ft R
MPI_Wtime returns a floating-point number of seconds, representing elapsed wall-clock time since some time in the past.
.PP
The "time in the past" is guaranteed not to change during the life of the process. The user is responsible for converting large numbers of seconds to other units if they are preferred.
.PP
This function is portable (it returns seconds, not "ticks"), it allows high resolution, and carries no unnecessary baggage. One would use it like this:
.sp
.nf
    {
       double starttime, endtime;
       starttime = MPI_Wtime();
        \&....  stuff to be timed  \&...
       endtime   = MPI_Wtime();
       printf("That took %f seconds\\n",endtime-starttime);
    }
.fi
.PP
The times returned are local to the node that called them. There is no requirement that different nodes return the "same" time.
.SH NOTES
The boolean variable MPI_WTIME_IS_GLOBAL, a predefined attribute key that indicates whether clocks are synchronized, does not have a valid value in Open MPI, as the clocks are not guaranteed to be synchronized.

.PP
This function is intended to be a high-resolution, elapsed (or wall) clock. See MPI_Wtick to determine the resolution of MPI_Wtime.
.PP
On POSIX platforms, this function may utilize a timer that is cheaper
to invoke than the gettimeofday() system call, but will fall back to
gettimeofday() if a cheap high-resolution timer is not available.  The
ompi_info command can be consulted to see if Open MPI supports a
native high-resolution timer on your platform; see the value for "MPI_WTIME
support" (or "options:mpi-wtime" when viewing the parsable
output).  If this value is "native", a method that is likely to be
cheaper than gettimeofday() will be used to obtain the time when
MPI_Wtime is invoked.
.PP
For example, on platforms that support it, the
.I clock_gettime()
function will be used to obtain a monotonic clock value with whatever
precision is supported on that platform (e.g., nanoseconds).
.PP
Note, too, that the MCA parameter opal_timer_require_monotonic can
influcence this behavior.  It defaults to true, but if set to false,
Open MPI may use a finer-grained timing mechanism (e.g., the
RDTSC/RDTSCP clock ticks on x86_64 platforms), but is not guaranteed
to be monotonic in some cases (e.g., if the MPI process is not bound
to a single processor core).
.PP
This function does not return an error value. Consequently, the result of calling it before MPI_Init or after MPI_Finalize is undefined.

.SH SEE ALSO
MPI_Wtick
.br

